This proposal concerns signaling mechanisms regulating cell orientation in the epithelial plane in the vertebrate embryo that is known as planar cell polarity (PCP). This common phenomenon is mediated by a group of conserved proteins that polarize to one side of an epithelial cell in the tissue. Secreted Wnt proteins have been implicated in the control of PCP, but whether and how Wnt proteins provide the directional input to the PCP system remains unclear. Our laboratory has a long-term interest in Wnt signaling and cell polarity, but mechanistic studies were hampered by lack of tractable models and suitable molecular markers. Recently, our group has developed specific antibodies and fluorescent sensors for PCP analysis and discovered a unique polarity of cells in Xenopus gastrula epidermis and the neural plate. The proposed studies will use the new tools to address the following key questions that continue to challenge the field. A. What mechanisms are responsible for core PCP protein segregation in polarized cells? B. What are the spatial cues or signals that control PCP? C. How does cell polarization translate into morphogenetic cell behaviors? Our experiments will uncover molecular mechanisms regulating polarization of the core PCP protein Vangl2 to the anterior apical edge of cells in the Xenopus. Other studies will clarify the role of Wnt ligands in this process and identify new proteins that function in PCP using mass spectrometry-based approaches. These experiments will use Xenopus embryos, a unique in vivo model, in which biochemical, embryological and cell biological approaches can be combined. The proposed studies will advance the knowledge of basic cell biological mechanisms underlying vertebrate early morphogenetic events, such as gastrulation and neurulation. Since misregulation of Wnt and PCP signaling has been causally linked to many congenital defects and syndromes, polycystic kidney disease and multiple cancers, these studies are highly relevant to human health.